1. Field of the Invention
The present invention relates generally to aluminum-lithium alloys, and in particular, to such alloys useful in the aerospace industry.
2. Description of Related Art
Aluminum-lithium alloys have long been recognized as an effective solution to reduce weight of structural elements because of the low density of these alloys. However, the different requirements of the aircraft industry materials such as, having a high Young modulus, high compression resistance, high damage tolerance and high corrosion resistance, have proven difficult to be obtained simultaneously. Al—Li alloys are particularly sensitive to crack turning or crack deflection, which is among the problems related to damage tolerance limiting their use. (Hurtado, J A; de los Rios, E R; Morris, A. J, <<Crack deflection in Al—Li alloys for aircraft structures”, 18th Symposium of the International Committee on Aeronautical Fatigue, Melbourne; UNITED KINGDOM; 3-5 May 1995. pp. 107-136. 1995).
Crack deviation, crack turning or also crack branching are terms used to express propensity for crack propagation to deviate from the expected fracture plane perpendicular to the loading direction during a fatigue or toughness test. Crack deviation happens on a microscopic scale (<100 μm), on a mesoscopic scale (100-1000 μm) or on a macroscopic scale (>1 mm) but it is considered detrimental only if the crack direction remains stable after deviation (macroscopic scale). The phenomenon is a particular concern for fatigue trials in L-S direction for aluminum-lithium alloys. The term crack branching is used herein for macroscopic deviation of cracks in L-S fatigue or toughness tests from the S direction towards the L direction which occurs for rolled products with a thickness of 30 mm or higher. Crack branching may occur in relation to the rolled product composition and microstructure and to the test conditions. Rolled products made of AA7050 can be considered as a reference of products having a low propensity to crack branching.
Crack branching has been considered as a major problem by aircraft manufacturers because it is difficult to take into account to dimension parts, thereby making impossible the use of traditional design methods. Thus, crack branching invalidates conventional, mode I based, materials testing procedure and design models. The crack branching problem has proven difficult to solve. Recently it was considered that in the absence of solution for avoiding crack branching, efforts should be directed to predicting crack branching behaviors. (M. J. Crill, D. J. Chellman, E. S. Balmuth, M. Philbrook, K. P. Smith, A. Cho, M. Niedzinski, R. Muzzolini and J. Feiger, Evaluation of AA 2050-T87 Al—Li Alloy Crack Turning Behavior, Materials Science Forum, Vol 519-521 (July 2006) pp 1323-1328).
There is a need for an aluminum lithium alloy rolled product for aircraft applications and in particular for integrally machined parts, which has a low propensity to crack branching.